A non ouabain-like inhibitor of the sodium pump in uremic plasma ultrafiltrates and urine from healthy subjects

A non ouabain-like inhibitor of the sodium pump was separated from uremic plasma ultrafiltrates and normal urine. Under the same chromatographic conditions (C-18 column and a gradient of acetonitrile as eluant), ouabain was eluted in a fraction different from the inhibitor. Affinity chromatography based on the formation of a complex between Na,K-ATPase and the inhibitor achieved the differentiation ouabain. Without magnesium and sodium phosphate, ouabain could not bind to enzyme whereas the inhibitor did. A study of Na,K-ATPase enzyme kinetics showed the inhibitor was not competitive for K+, which further differentiates it from ouabain. It was uncompetitive for ATP and seemed competitive for Na+. These results indicate that the inhibitor acts inside the cell, unlike ouabain, and thus its action mechanism appears to be original. (C) 1998 Elsevier Science B.V. All rights reserved.